thank you for the response! the connection I found is from Rotterdam and also the price is cheaper than the Train. I read it only takes me 15 minutes from Rotterdam to Delft. In the mean time, now you makes me wonder about Hague as well.

Going by train can be as cheap as 46,90 EUR by going over Groningen; see bahn.de (don't know if that search URL will keep working) - though it takes nearly 6 hours.

Actually, the cheapest train fare is normally 29euros, booked through the same Deutsche Bahn website, although it has to be booked in advance and their are limited number of tickets at that price. I have frequently used them. I have just put in Bremen-Delft on a random November date, and been offered several 29euro tickets during the day.

As previously noted, Touring/Eurolines singles (coach) are 26.60 each way on special offer, but that is only to Den Haag or Rotterdam (so you still need to pay to Delft, or be collected) and take a couple of hours longer than the train.

Having said that, if you can find a useful ride, Mitfahrgelegenheit - Ride sharing can easily be the cheapest, or certainly quickest. If lucky, you can get a ticket for free.

To answer your original question:

could you help me how to get the cheap connection.

The bottom line is to plan ahead, be flexible wit your days/times if possible, and book as far ahead as possible. That will get you the cheapest public transport option. Doing so, you should not be paying more than 60euros return.